Transition from the second-generation cellular phone system to the third-generation cellular phone system is now in progress. With such system transition, an integrated circuit fabricated by integrating a logic circuit, an amplification circuit, etc. with the switch circuit is employed in front-end portions of the cellular phones in increasing number.
In that type of integrated circuit, it is demanded to improve not only characteristics of the switch circuit alone, but also characteristics of the entire integrated circuit, such as an insertion loss and isolation. Therefore, some integrated circuit is constituted as a semiconductor switch device in which a depletion-type FET (hereinafter referred to as a “D-type FET”) and an enhancement-type FET (hereinafter referred to as an “E-type FET”) are formed on a single semiconductor substrate in a mixed way. For example, see Japanese Unexamined Patent Application Publication No. 2005-203642 (PTL 1). The D-type FET has a normally-on characteristic that a threshold voltage is negative when a drain current starts to flow, and it is featured in having a smaller insertion loss than the E-type FET. The D-type FET is used in many switch circuits. The E-type FET has a normally-off characteristic that a threshold voltage is positive when a drain current starts to flow, and it is used in many amplification circuits and many logic circuits.